Геологический сборник № 5. Информационные материалы

Ernst, R.E., Pease, V., Puchkov, V.N., Kozlov, V.I., Sergeeva, N.D., Hamilton, M. GEOCHEMICAL CHARACTERIZATION OF PRECAMBRIAN MAGMATIC SUITES OF THE SOUTHEASTERN MARGIN OF THE EAST EUROPEAN CRATON, SOUTHERN URALS,

ABSTRACT Vendian (ca. 650–600 Ma) Arsha formation. Underlying ArcheanHPaleoproterozoic and the Riphean complexes are The Bashkirian anticlinorium in the western slope intruded by dike swarms and sills. of the southern , Russia, exposes the poorly Determining the age of clastic sediments is difficult understood magmatic units of the southeastern margin because they generally lack fossils, however, the presence of the East European craton. Volcanic and intrusive of igneous rocks provides the opportunity for radiometric rocks were analyzed for major and trace elements (91 dating. Combining crossHcutting relationships with samples), and the data used to correlate volcanic suites and radiometric dating, it is possible to constrain the relative to correlate poorlyHdated intrusive suites with the volcanic timing and tectonic evolution of this margin. We have suites, and also assess geodynamic setting. Five distinct collected samples from the various suites of igneous geochemical suites were identified although their ages rocks (Figs. 1, 2) along the southeastern margin of the are not always wellHconstrained 1) Early Mesoproterozoic East European craton, to perform modern, highH (lower Riphean, type section) Ai formation volcanics resolution geochemical analyses in order to: (ca. 1650 Ma) situ, and compositionally similar sills 1) Identify a geochemical ‘fingerprint’ (mainly based on cutting the Early Mesoproterozoic (Lower Riphean) incompatible trace elements) for each volcanic and formation 2) Late Mesoproterozoic (Middle Riphean) intrusive suite. units including the Mashak volcanics, dykes of the Bakal 2) Use these geochemical fingerprints to suggest quarries cutting the Mesoproterozoic (uppermost Lower magmatic age correlations between intrusive and Riphean), the Berdyaush rapakivi pluton and crosscutting extrusive sequences. dykes (grouped as the Mashak Igneous Event) 3) Dykes 3) Use geochemistry to determine the tectonic setting and sills cutting basement of the Taratash complex as and mantle sources types for the various magmatic exposed in the Radashni quarry and a high Mg dyke in the suites. Bakal quarry. 4) dykes cutting Neoproterozoic (Upper In addition, we provide a new U–Pb age for middle Riphean) units and finally 5) Late Neoproterozoic Riphean dyke. (Vendian) units consisting of basalts, andesite and dacite lavas and tuffs of the Arsha formation.. A precise U–Pb age of 1385,3±1,4Ma for the Bakal dyke represents the most REGIONAL SETTING precise estimate available for the Mashak Igneous event. The Mashak event may represent the Mesoproterozoic The stratigraphy of the MesoH and Neoproterozoic breakup of the East European craton, and can be correlated sedimentary and volcanic formations of the region of with the Midsommerso sills — ZigHZag Dal volcanics the Bashkirian anticlinorium represents a type section in northern Greenland. for the Riphean (ca. 1650 to 650 Ma) (Keller & Chumakov, 1983). The Riphean overlies unconformably the ArcheanHPaleoproterozoic Taratash crystalline basement INTRODUCTION complex and is represented by mostly terrigenous and carbonate formations (Fig. 1). The Lower Riphean The southeastern part of the East European Craton (Burzyanian), with a lower age limit ca. 1650 Ma consists preserves a Mesoproterozoic (EarlyHMiddle Riphean) of Ai, Satka and Bakal formations (and ageHequivalent through Neoproterozoic (Late Riphean and Vendian) Bolsheinzer, Suran and Jusha formations in the southern volcanoHsedimentary history, interpreted to represent Bashkirian antinclinorium). The Middle Riphean a deep (up to 15 km) intracontinental rift basin and/or (Yurmatinian), with a lower limit ca. 1350 Ma comprises a longHlived Precambrian passive margin, affected by the Mashak, Zigalga, ZigazinoHKomarov, and Avzyan collision in the Late Vendian (Keller and Chumakov, formations. The Upper Riphean (Karatavian), with a lower 1983; Puchkov, 2000). limit of ca. 1000 Ma comprises the Zilmerdak, Katav, These Precambrian units are exposed along the Inzer, , Uk and (locally developed) Krivoluk western slope of the Urals in the region of the Bashkirian formations. The Riphean is overlain by the Vendian anticlinorium (Fig. 1), where magmatic rocks have a (Asha series) with a time span ca. 650–540 Ma, and relatively restricted geographic, chronological and volumetric which is divided into the Lower and Upper subseries and occurrence. Volcanic and volcanoHsedimentary rocks include a number of formations. The Lower and Middle Riphean the Lower Riphean (ca. 1650 Ma) Ai formation, Middle correspond approximately to the Mesoproterozoic, Riphean (ca. 1370 Ma) Mashak formation, and the Lower the Upper Riphean and Vendian to the Neoproterozoic,

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Figure 1. Schematic geological map of the Bashkirian meganticlinorium (Southern Urals, Russia). Modified from Kozlov (2002) Sampling sites: Radashny quarry (sites 1–4), Bolshoi mtn. (site 5), quarry at the southern slope of Maly Miass mtn. (site 6), main dike Bakal quarry (site 7), Irkuskan quarry (site 8), cutting Berdyaush pluton (site 9), along road to on southeastern outskirts of Kusa town (site 10), right banks of Kusa and Ai rivers (sites 11–13), Matveev Zalavok tract, Bolshoi Shatak range (site 14), Krutaya mtn, to the north of Tirlyan village (sites 15–17), Kapkatash mtn, Bolshoi Shatak range (site 18), Bolshoi Kliuch Creek, Bolshoi Shatak range (site 19), along road 2 km to the west from Mezhgorye town (site 20), along road 0.8 km to northHwest from Berdagulovo village (site 21), along road 1.2 km to the west from the highway bridge over Maly Inzer River (“Otkop” location) (site 22), in roadcut of a abandoned railroad line, 1.5 km to the S–E of Inzer railroad station at the left bank of Maly Inzer river (site 23). 10 km to the south of Ishlya village, Karagas range, Karagas #3 drill hole (site 24), Bolyshoy Kliuch (site 25), central Bainazarova along road between Beloretsk and Byrzyan (site 26), Belaya river, 10 km downstream of Nizhni Avzyan village (site 27). Structures mentioned in the text: I – Taratash anticline, II – Yamantau anticlinorium, III – Tirlyan syncline

120 Геологический сборник № 5. Информационные материалы and the Vendian is almost coeval with Ediacarian of the Volcanic episodes are associated with the Ai (Lower International Time Scale (Gradstein et al., 2004). Riphean), Mashak (Middle Riphean) and Arsha (Lower

Figure 2. The general scale of the Upper Precambrian of Russia (after Semikhatov et al., 1991) and generalized stratigraphic column of the Upper Precambrian of the axial part and western limb of the Bashkirian meganticlinorium: Site numbers located in Fig. 1 Formations of the Upper Precambrian of the Southern Urals: аi – Ai, st – Satka, b – Bakal,, ms – Mashak, zg – Zigalga, zk – ZigazinoHKomarovsk, av – Avzyan, zl – Zilmerdak,, kt – Ratav, in – Inzer, mn – Minyar, uk – Uk, kr – Krivoluk, bk – Bakeevo, ur – Uryuk, bs – Basinsk, kk – Kukkarauk, zn – Zigan. The Arsha formation is probably coeval with Bakeevo and Uriuk formations of this section. The age boundaries between the stratigraphic units of the Riphean and Vendian as are those approved by the AllHUnion Stratigraphic Meeting in Ufa, 1990 (Semikhatov et al., 1991). Since then, in early 2006, new versions of the base of Karatavian (1030 Ma) and base of the Vendian (600 Ma) were suggested in the 3rd Edition of the Stratigraphic Code of Russia. In the first case the change of the boundary is within the indicated error (±30 Ma). As for the second case, the age of the base of the Vendian is still very weakly constrained by the geological and geochronological data. It is also possible that this boundary is diachronous, getting older toward the open sea basin (i.e. in the direction of the Urals). Therefore, for the time being, we continue to use the earlier version of the boundary, which is not decreed, but generally accepted by the geological community

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Vendian) formations. In addition, diabase dikes and sills to zonal greenschist and amphibolite facies and are of probable Riphean age(s) intrude the ArcheanH known as Kuvash formation. Mashak formation volcanic Paleoproterozoic Taratash crystalline complex, and the rocks include effusives and subvolcanic intrusions of Riphean Satka, Bakal, Inzer or Krivoluk formations, basaltic to rhyolitic compositon. The most comprehensive and the middle Riphean rapakivi Berdyaush granite. section of Mashak formation is found in the No 3 Karagas borehole (Fig. 1, point 15). The borehole penetrates basic metavolcanics of effusive and volcanoclastic facies. Lower Riphean Ai formation VolcanoHsedimentary and sedimentary rocks are observed to alternate in an irregular way. Lower Riphean volcanic and volcanoHsedimentary Brecciated lavas (DunganHSungan mtn.) and some rocks are known from the northern part of the Bashkirian subvolcanic bodies are identified as the necks of ancient meganticlinorium, in limbs of the Taratash anticline volcanoes (Yamaev and Shvetsov, 1973). Parnachev (1981) (Fig. 1) which have the typical northeast trending strike concluded that Mashak volcanism and sedimentation of the modern Urals (e.g. Puchkov, 2002). The sedimentary occurred in grabenHlike (rift) structures (with NNE component of the Ai formation consists of coarse strike). terrigenous deposits, with conglomerates near the Acid volcanic rocks are recorded only in the lower bottom, close to the contact with the crystalline parts of the formation and comprise 10–15% of the total basement. Conglomerate pebbles and boulders consist thickness of volcanic rocks. They are represented by of quartzites, amphibolites, gneisses and other basement rhyolites, trachyHrhyolites and dacites. The rhyolites rocks (Kozlov et al., 1989). The volcanic rocks are were dated by the conventional multiHgrain U–Pb (zircon, situated in the upper unit of the Navysh subformation 1350±30 Ma) and Rb–Sr (whole rock, 1346±41 Ma) of Ai formation, and consist predominantly of effusive methods (Krasnobaev, 1986), which constrains the age rocks (lavas, lava breccias), but also include subvolcanic of the lower part of the Yurmatinian series. In addition, bodies and dikes (dolerite, and more rare dacite a Rb–Sr age of 1360±35 Ma was obtained for a dyke in porphyry) and volcanosedimentary rocks (tuff and the Bakal quarry (Ellmies et al., 2000), and a Pb–Pb tuffaceous units of different grain size). The Navysh lavas single zircon age of ca. 1350 Ma was obtained for an are predominantly metabasalt, and the Navysh dacite eclogite facies mafic dyke in the Beloretzk metamorphic porphyries have a U–Pb zircon date of 1615±45 Ma complex (Glasmacher et al., 2001). A comprehensive listing (Krasnobaev, 1986; Krasnobaev et al., 1992), interpreted of previous geochronology is provided in Fig. 2. as the time of crystallization, which constrains the age Recently more accurate ages have been obtained of the lower part of the Riphean deposits in their type from Berdyaush intrusive suite rocks, cutting Satka section. Previous K–Ar data from Navysh diabases formation, mainly by SIMS (Secondary Ion Mass range in age from 419–671 Ma and imply metamorphic Spectrometry) method (Ronkin et al., 2005). Specifically, resetting (Lennykh et al., 1978). Parnachev (1981) 1395±20 Ma was obtained from a gabbro (probably regards the Navysh formation as volcanoHsedimentary a xenolith, and perhaps representing an inclusion from graben facies formed in a marginal part of the KamaH a coHgenetic intrusion), 1372±12 Ma from a quartz Belsk (Mashak) paleorift which has a northwestern syeniteHdiorite, 1369±13 Ma from a rapakivi granite and strike, oblique to the modern structure of the Urals 1373±21 and 1368.4±6.2 Ma from a nepheline syenite. (Puchkov, 2002). Therefore, the age of the Berdyaush suite is ca. 1370. The Ai formation is overlain by terrigenousH carbonate deposits of the Satka and Bakal formations. Further south, along the rest of the Bashkirian megaH Lower Vendian (Arsha Formation) anticlinorium the terrigenousHvolcanogenic Ai formation is replaced by the age equivalent carbonateHterrigenous Lower Vendian volcanogenic rocks in the Southern Bolsheinzer formation. Although the analogues of the Urals occur locally in the northern part of the Tirlyan Lower Riphean are exposed in the axial part of the syncline (Fig. 1), (eastern limb of the Bashkirian Yamantau anticlinorium (Fig. 1) volcanogenic material meganticlinorium, 40–50 km to the north from town is absent there. of Beloretsk). These volcanic rocks are situated in the middle part of the Arsha formation in association with terrigenous rocks, tillites among them. The Arsha volcanics Middle Riphean (Mashak Fm and correlatives) include effusive, explosive and volcanoHsedimentary facies. Andesitic and rare daciteHandesitic lavas and The Middle Riphean volcanic and volcanoH brecciated lavas predominate (Parnachev and Kozlov, 1979, sedimentary rocks of the Mashak formation, form Alekseyev, 1984, Parnachev, 1981). Andesites of Krutaya a northeast trending belt (parallel to the typical Uralian mtn. were dated by the Rb–Sr method as 677±31 Ma strike (20°–30° NE), more than 100 km long and 1–12 km (Gorozhanin, 1995). According to Parnachev (1981), wide. In the northern part of the Bashkirian anticlinorium the Arsha complex belongs to the NNWHtrending the analogues of Mashak formation are metamorphosed KamaHBelsk paleorift structure.

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PREVIOUS GEOCHEMISTRY geochemistry (34 analyses) and geochronology (mostly K–Ar), they identified three groups of diabases. The oldest According to Parnachev (1981), the least altered group (ca. 1650 Ma) represents tholeiitic diabase of trachybasalts of the Early Riphean Ai formation (23 composition similar to “traps” (flood basalts) of ancient chemical analyses) have high TiO2 (2.74%), total alkalis platforms. Tholeiitic diabase of the second group has (6.69%) and P2O5 (0.79%), while Ni/Co ratios and ages of 1250–1150 Ma, and based on alkali and titanium the distribution of REEs are comparable to trachybasalts content and alkali/alumina ratio correspond to “traps” of the East African rift zone. (flood basalts) of young platforms. The third group Metabasalts of the Middle Riphean Mashak and comprises intrusions of essexiteHdiabase with ages of correlative Kuvash formations correspond to olivine 670–420 Ma. Taking into account the widespread regional tholeiites with low contents of Cr (70 ppm), Ni (70 ppm), alteration affecting all suites (discussed earlier), it is unlikely Ba (210 ppm), Sr (100 ppm), and REE (90 ppm), and high that these K–Ar dates are reliable. contents of V (285 ppm) (Parnachev, 1981; Parnachev To summarize, previous geochemical studies have et al., 1986). These metabasalts are part of a biHmodal established that three ages of volcanic rocks are present association characteristic of continental rift complexes (lower Riphean, middle Riphean and lower Vendian) and (Grachev, 1977). A more recent trace element study by that each has geochemical similarities with extensional Karsten et al. (1997) interpreted metabasalts of the Mashak volcanism. There are also several intrusive suites formation as tholeiites of rather complex character. recognized. However, a strong link between the intrusive Concentrations of Zr, Y, Nb, Ti, are lower than typical and extrusive is difficult to make based on the available basalts of continental rifts, but higher than those in the K–Ar and trace element data. Apart from the recent study oceanic rifts (NMORB). The absence of an EuHanomaly by Karsten et al. (1997) on the Mashak and related Kuvash suggests mantle genesis of primary magmas and no formation, the available trace element data is very old. fractionation of plagioclase. According to Karsten et al. Therefore, it is important to reevaluate the geochemical (1997), metabasalts and metarhyodacites of Mashak setting of all the magmatic suites in the Bashkirian formation have differing geochemical characteristics and anticlinorium using a modern trace element dataset. therefore do not belong to a single biHmodal magmatic association; the metabasalts are linked to active rifting while metarhyolites situated closer to the lower part of SAMPLING SUMMARY AND the volcanic section could be formed in an intraplate METHODOLOGY setting prior to, or at an early stage of rifting. Lower Vendian (see the comments to the Fig. 2) Arsha Samples were collected from various magmatic metabasalts (Parnachev and Kozlov, 1979; Parnachev, suites through sections of the Lower Riphean Ai and Satka 1981 his table 1, 29 chemical analyses) have high TiO2 formations, the Middle Riphean Mashak formation, and (up to 3%), Fe oxides (up to 18.5%), P2O5 (up to 1.53%), the Vendian Arsha formation (Fig. 2). In addition REE concentrations, Ni/Co ratios (0.15). These samples were collected from dykes and sills intruding chemical features combined with a trachytoid type of the ArcheanHPaleoproterozoic and Lower Riphean matrix texture classify them as trachybasalts. Taratash, Bakal, and Satka formations, as well as Upper Lennykh and Petrov (1978) studied dykes cutting Riphean Inzer and Krivoluk formations. Site information the ArchaeanHPaleoproterozoic metamorphic rocks of is summarized in Table 1 and 2 and details of locations the Taratash complex at the Radashny quarry. Based on are given Figs.1&2. Table 1 Location and descriptions of extrusive samples (more details are given in Figs.1&2)

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Table 2 Location and descriptions of intrusive samples (more details are given in Figs.1&2)

All geochemistry samples were cleaned of surface percent). Trace elements (including REE) were analyzed alteration by hydraulic splitting followed with sandblasting. by ICP–MS and are reported in parts per million (ppm). Geochemistry samples were submitted to Activation Calibrations were made using reference samples and Laboratories, Canada for milling and analysis. All samples international standards. Relative standard deviations are were milled in “mild steel” (RX2 procedure) which ~1% for SiO2 and ~2% for the other major elements, produces only minor iron contamination (<0.2%). Major except MnO and P2O5 (±0.01%) and K2O (±0.005%). and trace elements were analyzed using the “4Litho” Relative standard deviations are generally better than package. Major element oxides were determined by about 5% for most trace elements. Detection limits lithium metaborate fusion using 0.2 g of whole rock powder of particular interest include the high field strength and the ICP–ES technique (results reported in weight elements (Hf = 0.1, Nb = 0.2, Ta = 0.01, Th = 0.05, U =

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0.01, light REE = 0.05, middle and heavy REE = 0.01). set apart from the other samples. In site 18, sample 04 The determination of structurally bound H2O was made is anomalous. At site 19, sample 06 is anomalous. via loss on ignition (LOI). The analytical data are Site 24 does not show a simple grouping; a dominant presented in Table 3. Data was normalized to 100% group is represented by samples 37.5, 40, 51, 260, 280, volatileHfree before plotting. 296, 311, 513 and 514. Samples that are clearly anomalous include sample 477, and possibly 210 and 224 Samples 394 and 399.5 match each other but have distinctly GEOCHEMISTRY steeper LREE from the other samples (and have high LOIs of about 4.5%). Various major and trace element compositions can A similar complex situation is observed for the be useful for distinguishing between magmatic events intrusive suites. Sites 1–4 in the Radashny quarry have and assessing the contribution of various mantle a main group defined by samples 0201, 0202, 0203, components (e.g. Condie, 2003 and references therein). 0401, 0402. Both samples from site 3 are anomalous to This is best achieved with pristine samples representative this group and to each other, and sample 0403 is also of primitive melt compositions, i.e. — fresh samples anomalous. In site 10, sample 06 is anomalous; samples unaffected by crystalHmelt processes (crystal cumulates, 1–5 are from the main sill while sample 6 is from a thin crystal fractionates, etc.) which can obscure mantle (0.5 m) satellite sill located nearby but above the main processes/components. HFSE and REE are least mobile sill. The rest of the intrusive data is relatively consistent during metamorphic processes (e.g. amphibolite facies — within a site. Polat et al., 2003; blueschist facies — Mocek, 2001; eclogite Consequently, we eliminate those samples with facies — Becker et al., 2000). Consequently, these very high LOI (>5.5%): 24–477, 24–257.5, 24–513, elements are more likely to preserve unaltered geochemical 1803, 1605, 0801, and 0802, and anomalous samples signatures and can be used to identify mantle reservoirs (identified above), 14–01, 14–08, 16–05, 18–04, 19–06, in altered samples. Each of these factors is evaluated and 24–210, 24–224 and 24–477, from subsequent analysis. potential geochemical ‘fingerprints’ for the igneous suites are presented below. Geochemical Diagrams

Identification of altered and anomalous samples The samples are mainly mafic, but with some felsic components (sites 6, 16, 17, 24–185, and selected samples Most of the analyzed samples have LOIs less than from site 14: samples 1, 9, 10 and 11) (Fig. 4). To distinguish 3%, though some have values up to 10%. (Fig. 3). Since felsic samples in subsequent figures, we enclose their fresh, unaltered basalts generally have LOI<3 wt %, symbols with an open circle. The extrusive and intrusive those high LOI values may be evidence of subsequent suites are considered separately and are each assessed alteration. However, specific suites have higher LOIs using a variety of diagrams: standard classification which seem to be characteristic of the units: Ai formation diagrams, trace element setting diagrams and mantle (lower Riphean) volcanics (units 5 and 6); Middle source diagrams (e.g. LeBas, 1986; Condie, 1997, 2001; Riphean rocks (units 14, 18 and 24, but not unit 19), Tomlinson and Condie, 2001; Rudnick, 1995; Herzberg, and sills; and dykes cutting the Satka formation (units 1995; Rollinson, 1993; Pearce, 1996). We first provide 10–13). In addition, two of the four high MgO samples an overview using the TAS diagram (Fig. 4), REE diagrams from the intrusive unit 8 have high LOI. The lower (Fig. 5), multiHelement diagrams (Fig. 6), AFM diagram Riphean volcanics and intrusives cutting lower Riphean (Fig.7), TiO2vs Mg# (Fig.8), and Zr/TiO2 vs Nb/Y (Fig.9). group up to just over 5% LOI. So as to avoid splitting Among these (as discussed above) the trace element these apparent natural groups, we take the high LOI diagrams are least susceptible to the effects of alteration. cutoff as 5.5%. Additional diagrams that are useful for tectonic setting Many Middle Riphean samples have very low K include Zr vs Ti vs Y (Fig. 10), MnO2 vs TiO2 vs P2O5 (Figs. 4a and 7). This may be partly characteristic of the (Fig. 11), La vs Y vs Nb (Fig. 12), and Ti vs V (Fig. 13). suite, but may represent systematic removal of K (and Ba These diagrams can basically distinguish midHocean and Rb) during the low grade metamorphism that ridge basalts (MORB), from volcanic arc (VAB), from affected the suites. In most cases the alteration has not within plate basalts (WPB), and there are subcategories: significantly affected the patterns of high field strength thus MORB and WPB can divide into tholeiites and trace elements (e.g. REE on Fig. 5), and those data can alkali basalt groups, VAB can divide into tholeiites, calcH be used to define groupings with which to recognize alkaline basalts and shoshonitic, and both VAB and WPB anomalous samples. The data from sites 5 and 6 are well can also divide into oceanic and continental types (Pearce, grouped. Site 14 has two groups, samples 2–7 representing 1996). These diagrams as well as Figs. 14–18 are useful the more mafic group and 9–11 being more felsic, and for developing geochemical ‘fingerprints’ for the various the remaining sample (no. 1) falls outside both groups magmatic suites, and Figs. 16–18 can also be used to and is considered anomalous. In site 16, sample 05 is assess mantle source reservoirs (after Condie, 2003).

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Figure 3. Loss1on1ignition (LOI). Legend for symbols used in this and other geochemical figures. Felsic samples here and on subsequent plots are shown in open circles

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Figure 4. Total alkalis versus SiO2 (TAS) volcanic rock classification diagram (after LeBas et al., 1986). A) Volcanic rocks. B) Intrusive rocks superimposed on TAS for comparison to possible extrusive equivalents

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Figure 4. Total alkalis versus SiO2 (TAS) volcanic rock classification diagram (after LeBas et al., 1986). C) Na2OvsK2O Symbols as in Fig. 3.

The La/Nb ratio (Fig. 14) is a key discriminant of 2001, 2003). Mantle components include MORB, EM1, the subduction (recycled) component in the mantle. Values EM2, HIMU, FOZO (Hart, 1988; Hart et al., 1992), for this ratio greater than 1.4 distinguish basalts erupted and also potential crustal and lithospheric contamination. at ocean ridges, ocean islands and oceanic plateaus from Originally identified using isotopes (e.g. Nd, Sr, Pb, Hf), those erupted in arc environments because Nb largely these components can also be recognized in terms of remains in the descending slab, whereas La is carried their trace element compositions. More recently, Condie into the hydrated mantle wedge during subduction (2003) has grouped the deep mantle reservoirs into the (Rudnick, 1995). The hydrated mantle wedge, which is following end members (on trace element diagrams, the source of most subductionHrelated magmas, has high Figs. 16–18): DEP (a depleted source similar to MORB, La/Nb. Basalt with ratios >1.4 can reflect an arc source, but located in the deep mantle — probably equivalent contamination of the lithosphere by arc sources, or to FOZO). REC is near average OIB (EM1 and EMII) reflect partial melting of the subcontinental lithospheric and HIMU, and are assumed to represent recycled mantle. Plotting La/Nb against Hf/Sm (Fig. 14) lithosphere, and EN is near continental crust and minimizes the effects of i) partial melting because presumably reflects crustal materials carried to depth in incompatible high field strength elements are insensitive the mantle. Figure Nb/Y vs Zr/Y (Fig. 18) can also be to ‘large’ degree partial melts, as represented by most used to assess whether or not a plume source is involved tholeiites, and ii) restite garnet because Hf is less based on a discriminant based on Icelandic data (Fitton incompatible than Sm in garnet; if garnet is present in et al., 1997). the source, low Hf/Sm ratios will be generated during partial melting. La/Sm versus Gd/Yb (Fig. 15) compares the slopes of heavy and light REEs. Volcanic Suites The La/Yb ratio summarizes the overall REE slope, which varies with the degree of partial melting The volcanic formations define distinct geochemical when garnet is left in the residue. The Th/Ta ratio groups, e.g. have geochemical fingerprints. The lower minimizes the effect of garnet fractionation and tends Riphean (ca. 1650 Ma) Ai formation is represented by to be slightly enriched in plume related sources. Plotting medHK calcalkaline trachybasalts (site 5) and dacites these two ratios against each other (Fig. 16) can be used (site 6). Both mafic and felsic sites have steep REE slopes, to discriminate mafic magmatic suites, and can also be with enriched chondrite normalized LREE/HREE used to recognize mantle components (Condie, 1997, [(La/Yb)n = 7–13 for mafic and 13–15 for felsic samples],

135 Институт геологии Уфимского научного центра РАН a pronounced negative La–Nb anomaly), and a negative The middle Riphean (ca. 1370 Ma) Mashak events Sr anomaly. The mafic samples also have a positive K (sites 14, 18, 19, and 24) are characterized by lowH to anomaly. The Ai formation defines a geochemical mediumHK, mainly tholeiitic basalts (Fig. 3). This suite fingerprint (Figs. 14–16 and Table 4) with high La/Nb is characterized by a negative Sr anomaly, and negative K (2.2–3 and 4.2–4.6 for mafic / felsic samples) and low and Ba anomalies, though this is probably a consequence Hf/Sm ratios (0.6); moderate Th/Ta (2.5–4 and 11 for of the widespread low grade metamorphism. REE patterns mafic / felsic samples) and high La/Yb ratios (11–19 are moderately enriched chondrite normalized LREE/ and 20–23 for mafic / felsic samples), moderate La/Sm HREE, (La/Yb)n = 2–4, and there is a weak Ta–Nb (4–5 and 7 for mafic / felsic samples) and high Gd/Yb anomaly. With respect to geochemical fingerprinting ratios (2.8–3.0 and 2.2–2.6 for mafic / felsic samples). (Figs. 14–16), the Mashak magmatic events have low In terms of tectonic setting they plot as withinHplate and La/Nb ratio ranging from 0.8–1.5, low Hf/Sm ratios (0.8), alkaline affinities (Figs. 10–13) and in terms of mantle low to moderate Th/Ta (2–3) and La/Yb ratios (mainly sources they plot as a mixture of Recycled and Enriched 3.3–6), La/Sm (2–4) and low Gd/Yb ratios (1.5–1.9). sources (Figs. 16–18), and the negative La–Nb anomaly In terms of tectonic setting they plot as withinplate is likely due to lithospheric contamination rather than a (Figs. 10–13) and in terms of mantle sources they plot subduction component. as Depleted and significant addition of Enriched and

Figure 5. REE data normalized by chrondritic values

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Recycled sources (Figs. 16–18). More felsic samples they plot mostly in the withinHplate field and in terms from site 14 have a distinctly different pattern consisting of deep mantle sources they plot near the REC source. of negative K, Ba,m Rb, Sr P and Ti anomalies. The Vendian (ca. 600 Ma) Arsha formation is distinct from the older formations in having higher SiO2 Intrusive Rocks and lower MgO. These lowH to mediumHK rocks are broadly andesitic, but with Na2O+K2O and SiO2 negatively Dykes and sills also form distinct geochemical correlated (Fig. 4). The Arsha formation has the steepest groups (Figs. 4–19). Units cutting basement in the REE slopes with very enriched chondrite normalized Radashny quarry are scattered on some diagrams. Most LREE/HREE, (La/Yb)n = 9–20. Its geochemical typically, they exhibit moderate to high LOIs (1.5–5 wt%), fingerprint (Figs. 14–16) is defined by low La/Nb (below low TiO2, intermediate Mg# (30–50), have a variable the critical value of <1.4), scattered Hf/Sm ratios (0.4 & 1.2), REE slopes, (La/Yb)n 1–13, a strong subduction signature variable to low Th/Ta (1.3 & 4.3) and high La/Yb ratios (La/Nb 2.2–5.0), and minor negative P anomalies (Fig. 6). (13 & 20–30), high La/Sm (6.5–8.5) and moderate to high The high Mg site 8 which cuts the Bakal formation has Gd/Yb ratios (1.7 & 2.9). In terms of tectonic setting a similar composition to the Radashny quarry units,

in Sun and McDonough (1989). Symbols as in Fig. 3

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Table 4 Geochemical Summary of Extrusive Suites

although it is more primitive (Mg# >75). It also has low distinct ages (Middle Riphean and Upper Riphean or TiO2 (0.5–1%), high La/Nb (2.9), La/Sm is 5, Gd/Yb younger). They consist of intrusions cutting lower is 1.7, high Th/Ta (10–13), high La/Yb (8), low Nb/Th Riphean units: dykes (sites 20 and 21 cut the Suran (2), moderate Zr/Nb (13). Like sites 1–4 it also has formation which is considered an equivalent of the Satka a negative P anomaly. Considered as a group (sites 1–4 formation. Dyke (site 7) also cuts the Bakal formation and site 8), they are calcHalkali (Figs.7&9) and in terms (and is dated at 1385,3 Ma, see below). There are several of mantle sources they show a wide scatter around the deep intrusions constrained to be equivalent to or younger mantle Depleted and Enriched components (Figs. 16–18). than Middle Riphean. Site 25 cuts middle Riphean The sills (sites 10–13) which cut lower Riphean strata, and site 9 cuts the Berdyuash rapakivi pluton Satka formation (including the Kusa sill) define a second which is dated as ca. 1370 Ma. The second group group. They have moderate to high LOIs (3–5.5 wt%), plot consists of Sites 22, 23, 26 and 27 which cut upper as basalts near the trachyHbasalt boundary (Figs. 4 & 9), Riphean. Broadly speaking all these sites have low LOI and have intermediate TiO2 (1.2–2.4 wt%) and Mg# (2–2.7%), low La/Sm (0.8–3), a range in Gd/Yb (1.9– (35–50) (Fig. 8), high La/Nb (2.2–3.0). In terms of 2), moderate to high TiO2 (1.5–3 wt%), lowHmoderate tectonic setting they define a predominantly within plate La/Nb (~1), and a range in Th/Ta (1.1–5). Within this setting, although the La/Nb value (>1.4) suggests group sites 9 & 20–21 are compositionally similar to the a subduction component. In terms of mantle sources, they dyke of site 7 which has known middle Riphean age. plot between the REC and EN fields (Figs. 16–18). Site 25 has a distinctive decrease in the more mobile The remaining intrusions plot together, but based elements, a pattern which is also present to a lesser on crossHcutting relationships must represent at least two degree in upper Riphean sites 26 and 27 (Fig. 6). Upper

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Table 5 Geochemical Summary of Intrusive Suites

Riphean sites 22 and 23 have the flattest REE patterns (22 & 23) are geochemically similar to the middle and may represent a third geochemical subgroup. In Riphean Mashak volcanics, though stratigraphically, it terms of tectonic setting, these define a within plate is clear that some of the dykes in this group must be (Figs. 10–13). In terms of mantle sources, the data plot Upper Riphean or younger. Therefore, this group must with Depleted signature with minor Recycled and define a second event which is indistinguishable Enriched contributions. In most diagrams these sites geochemically from the Middle Riphean group.

139 Институт геологии Уфимского научного центра РАН

Figure 6. Spider diagrams using normalization

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values in Thompson et al. (1983). Symbols as in Fig. 3

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Figure 7. AFM classification diagram after Irvine and Baragar (1971). Symbols as in Fig. 3

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Figure 8. TiO2 vs Mg#. Symbols as in Fig. 3

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Figure 9. Zr/TiO2 vs Nb/Y classification diagram of Winchester and Floyd (1977) as revised by Pearce (1996). Symbols as in Fig. 3

144 Геологический сборник № 5. Информационные материалы Figure 10. ZrFigure vs Ti/100 vs and Cann (1973). Symbols as in Fig. after Pearce Y*3 ternary classification diagram 3

145 Институт геологии Уфимского научного центра РАН *10 ternary classification diagram after Mullen (1983). Symbols as in Fig.*10 ternary classification diagram 3 5 O 2 vs P 2 Figure 11. MnO*10Figure vs TiO

146 Геологический сборник № 5. Информационные материалы Figure 12. LaFigure vs Y vs after Cabanis and Lecolle (1989). Symbols as in Fig. Nb ternary classification diagram 3

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Figure 13. Ti vs V after Shervais (1982). Symbols as in Fig. 3. IAT is island arc tholeiite, MORB is mid1ocean ridge basalt, BAB is back arc basin basalt

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Figure 14. La/Nb ratio for each suite, useful for identifying subduction signature (after Condie, 2003). Symbols as in Fig. 3

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Figure 15. Gd/Yb, versus La/Sm. Ratios used to compare the slope of the heavy rare elements against the slope of the light rare earth elements. Symbols as in Fig. 3

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Figure 16. Log Th/Ta vs log La/Yb. After Condie, 1997, 2003 and Tomlinson and Condie (2001). DEP (depleted), REC (recycled), EN (Enriched). OIB is ocean island basalt, PM is primitive mantle, UC is upper crust. NMORB is normal mid1ocean ridge basalt. OPB is ocean plateau basalts, MORB is mid1ocean ridge basalt, ArcB is arc basalt. Symbols as in Fig. 3

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Figure 17. Log Zr/Nb vs Nb/Th after Condie (2003). Symbols as in Fig. 3 and labels as in Fig. 16. HIMU is high ?

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Figure 18. Log Nb/Y vs log Zr/Y (after Fitton et al., 1997 as modified by Condie, 2003). Symbols as in Fig. 3 and labels as in Fig. 16

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GEOCHRONOLOGY fingerprinting. In addition a precise 1385 Ma U–Pb age allowed correlation of dyke 7 with the Mashak Igneous A U–Pb baddeleyite age was obtained for the Bakal Event. These correlations and new geochronology allow dyke (site 7) (Fig. 20, Table 6). Two baddeleyite fractions, the recognition of 5 distinct magmatic events in the region which are only slightly discordant, yield a precise age of (Table 7): Also these events can be classified in terms of 1385,3±1,4 Ma. This confirms a previously determined, but tectonic setting based on various classification diagrams less precise Rb–Sr age of 1360±35 Ma (Ellmies et al., (Figs. 10–14) and mantle sources (Figs. 16–18). The latter 2000). This age is broadly similar to Berdyaush dates and include estimates of deep mantle sources: Depleted, slightly older than the most recent estimates for the age of Recycled, and Enriched, and the significance of these the Mashak formation (see earlier discussion). diagrams is further evaluated in the discussion section. Event 1: The oldest magmatic suite is represented DISCUSSION by the volcanic rocks of the lower Riphean Ai formation (sites 5 and 6). Sills (sites 10–13) are very similar Comparison of felsic and mafic units geochemically (Tables 4 & 5), i.e. in terms of high LOI, of interpreted bimodal suites REE slope, multiHelement diagrams, and the various trace element classification and mantle source diagrams. These The Ai formation has a bimodal composition (c. f. sills cut the Satka formation, which is lower Riphean in mafic samples from site 5 and felsic samples from site 6 in age, but overlie the Ai formation and are therefore at Fig. 4). On REE diagrams (Fig. 5) and multiHelement least slightly younger. Pending geochronology, we tentatively diagrams (Fig. 6) the patterns are parallel suggesting that conclude that the sills cutting the Satka formation are the two mafic and felsic groups are related, and that they also lower Riphean in age, but (on stratigraphic grounds) probably derived by different degrees of differentiation of must be at least slightly younger than the Ai formation the same source magma. volcanics. For the middle Riphean units however, there is Event 2: The next magmatic group is represented a difference between mafic and felsic units, as noted by middle Riphean volcanic and intrusive rocks. These previously by Karsten et al. (1997). This is most clearly include the volcanic units (sites 14, 19, 24) and also the seen for site 14 (e.g. Fig. 6a) where three felsic samples dyke from site 7 (dated herein as 1385 Ma). Other dykes have numerous elemental anomalies (e.g. strongly negative cutting lower Riphean and middle Riphean units and Ti, P and Sr) that are not present (apart from a minor which are compositionally similar (Tables 4 & 5), most negative Sr anomaly) in the mafic samples. At the same importantly in REE slope, multiHelement diagrams, time we know that both felsic and mafic magmatism is very and various trace element and mantle source diagrams) similar in age. Therefore, the origin of felsic component is include sites 9, 20, 21, and 25. We correlate these units probably due to crustal melting under the influence of into the middle Riphean “Mashak igneous event” ascending basaltic magma (e.g. Bryan, 2002). (Ronkin et al., 2005). We next consider the age relationship with the Event 3: The next magmatic suite includes dykes Berdyaush pluton and a dyke which crosscuts this pluton. cutting the basement rocks in Radashny quarry (sites The nepheline syenite phase of the Berdyaush pluton yields 1–4). These have a strong compositional similarity a 1368.4±6.2 Ma age, and the crosscutting dyke (site 9) (Table 5) with the highHMg basalt cutting the Bakal geochemically matches the Bakal dyke dated herein as formation (site 8). On this basis we define a separate 1385,3±1,4 Ma. So, if this geochemical match (between sites event with an age of Lower Riphean or younger (sites 9 and dated site 7) is significant, then both the Berdyaush 1–4 and 8). pluton and its crosscutting dyke would belong to the Event 4: The fourth group is defined by intrusions Mashak event and indeed the felsic magmatism may cutting upper Riphean units. precede some of the mafic magmatism, as can be observed Event 5: The fifth and probably youngest geochemical in other bimodal or dominately felsic magmatic suites (e.g. grouping identifies the magmatic suite of the Arsha Bryan, 2002). formation (site 19). It is Vendian in age and consists of The Vendian suites (sites 15, 16 and 17) are all mostly felsic and intermediate rocks. These have trace broadly felsic (Fig. 4) and are geochemically distinct from element signatures distinct from event 4; for this reason the upper Riphean and lower Vendian dykes. However, we consider the two events to be separate. However, until precise ages are obtained for these younger suites, if events 4 and 5 were determined to be coeval, then comparison of felsic Vendian volcanism with mafic the difference between mainly felsic (event 4) and mafic intrusives remains uncertain. (event 5) compositions, could be a consequence of different coeval sources. If the felsic rocks are generated by Correlating intrusive and volcanic magmatism melting of continental crust caused by mafic magmatism, based on geochemical fingerprints then the composition of the felsic suite need not have geochemical similarity with the mafic rocks (Bryan In the previous section we noted correlations between et al., 2002). The absolute ages of events 4 and 5 remain intrusive and extrusive suites based on geochemical to be determined.

154 Геологический сборник № 5. Информационные материалы Table 6 Table U–Pb isotopic data for Main (diabase) Dyke, Central Quarry of Bakal data for Main (diabase) Dyke, Central U–Pb isotopic

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Table 7 Magmatic groupings based on new geochemistry and geochronology presented in this paper

Geodynamic Context (Tectonic Setting, mantle sources) day Urals (Puchkov, 2002). In particular it is inferred that the Ai formation sits on a shoulder of this aulacogen. Lower Riphean (Event 1 in Table 7): There are Based on the geochemical similarity we consider the sills conglomerates at the base of the Ai formation, overlying cutting the Satka formation to also belong to this the ArcheanHPaleoproterozoic crystalline complex with interpreted rift event (although a slightly later stage). angular unconformity and suggest a probable rift However, it cannot be excluded that the strong similarity relationship for the corresponding event. The Ai formation is accidental and that the sills were emplaced much later. with its associated mafic volcanics is lower Riphean and is The geochemistry of both the Ai formation volcanics and no older than about 1650 Ma. The geographic distribution the sills is consistent with a within plate setting with slight of the magmaticHsedimentary units of the Ai formation alkaline affinity. However, presence of a strong Ta–Nb is difficult to determine because of limited outcrop. anomaly indicate subduction character, or alternatively, However, the series of lower Riphean units is different in the involvement of lithospheric mantle. Mantle source the north than in the south: In the north the Lower Riphean diagrams favour a combination of Recycled and Enriched consists of the Ai, Satka and Bakal formations (Fig. 2). sources, and no plume signature is seen. In the south it consists correspondingly of Bolsheinzer, Event 2: Mashak event: In contrast the ‘grain’ of Suran and Jusha formations, with some differences of the Mashak formation is parallel to the present Urals, lithology, compared to the first three (Bolsheinzer lacks and therefore differs from that of Event 1, which is conglomerates and volcanics in the lower part of the evidence of a changed geodynamic setting between section and contains carbonates in the upper; Jusha is Events 1 and 2. The Mashak formation sediments substantionally more terrigenous). Ai formation volcanic (including basal conglomerates) and mafic volcanic rocks rocks are contemporaneous with the formation of a riftH define a rift setting and have dates of 1370–1380 Ma like NNWHtrending KamaHBelsk paleorift in the adjacent (see regional setting section). Several dykes (sites 7, 9, 20, platform, which has trend oblique to that of the present 21 and 25) are provisionally correlated with the Mashak

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Igneous event. The setting appears to be Within Plate, on paleomagnetism but also consistent geological but could also be back arc basin. There is no subduction correlations between the two (Buchan et al., 2000) signature. The mantle source is dominantly Depleted (Fig. 20). The position of the Mesoproterozoic Uralian with a substantial contribution of Enriched and Recycled margin of Baltica is shown (Fig. 20) and it is considered components. A plume source is indicated. that this margin was a longHlived passive margin after an Event 3: Lower Riphean or younger: This magmatic initial Mesoproterozoic or Neoproterozoic breakup (Pease event consists of the dykes and sills cutting the Radashny et al., 2006). In this reconstruction, the Midsommerso — quarry, and also the high Mg dyke cutting the Bakal ZigHZag Dal magmatism of northern Greenland (Upton formation. The only age constraint is that this event must et al., 2005) is close to the potential Mesoproterozoic be equal to or post lower Riphean. In terms of tectonic breakup margin. As one of the variations on the setting, an arc setting is strongly suggested by the consistent reconstruction we suggest that this margin brokeHup at calcHalkalic signature, and large negative La–Nb anomaly. 1380 Ma. Consequently, remnants of this magmatism Various tectonic diagrams support an arc setting, and there are present at the southern end of the Southern Urals is a strong subduction signature. Mantle sources include and at the northern end in northern Greenland. This subHequal contributions from Depleted and Enriched model would predict additional remnants of 1380 Ma sources, but a plume signature is not apparent. magmatism along this margin. However, its recognition Event 4: upper Riphean or younger: An additional may be complicated by poor exposure of preHUralian magmatic event is represented by intrusives (dykes) basement rocks. which cut upper Riphean units. This composition of this magmatism is distinct from the Vendian Arsha formation and thus provisionally represents a separate event. CONCLUSIONS Interestingly this magmatic event has composition similar to the middle Riphean event (Event 2) on many A new geochemical study of magmatic suites along diagrams (as discussed earlier) but clearly is distinct given the southeastern margin of the East European craton in that its members cut upper Riphean units, and unlike the southern Ural Mountains allows characterization of Event 2 have no subduction signature. The setting is within 8 volcanic sites, corresponding to 3 volcanic suites, and plate and there is not subduction signature. A Depleted 19 intrusive sites (in 11 sills and 8 dykes). Geochemical source with minor Recycled and Enriched contributions ‘fingerprints’ allow correlation of intrusive and volcanic is observed. The presence of a plume is equivocal. units into at least 5 different magmatic events. Event 5: Vendian: The felsic volcanics of the Arsha The earliest event includes the lower Riphean formation are Vendian in age. Their geochemistry (ca. 1650 Ma) Ai formation volcanics, for which suggests a within plate setting, and there is no subduction geochemistry indicates a within plate slightly alkaline signature (La/Nb < 1.4). The main mantle source is character, but also a strong La/Nb anomaly (and negative Recycled, and a plume source is implied. Ta–Nb anomaly) suggestive of subduction character and Summary: The data is consistent with early and mid mantle sources are mixtures of Enriched and Recycled Riphean rifting events (events 1 and 2). However, it is components, consistent with involvement of lithospheric uncertain whether one or both events led to rifting. and asthenospheric sources. The distribution of volcanic Event 3 (during post midHRiphean) is arc related. Late or rocks and sediments, including the presence of associated postHRiphean magmatism (event 4) also has rift/plume conglomerates supports a rift origin and a link with the signatures. Finally a Vendian event (which may be the KamaHBelsk paleorift. Several sill complexes, including felsic equivalent of event 4) has within plate signatures the Kusa sill cutting the Satka formation, represent and may be associated with a final rifting event. a discrete intrusive geochemical group which can be matched with the Ai formation although on stratigraphic grounds must be at least slightly younger. This first event Possible Link with 1380 Ma Large Igneous Province has withinHplate plus alkaline affinities; subduction remnant in northern Greenland signature, interpreted to be imparted by interaction with lithosphere. Mantle components are dominantly Enriched Intraplate magmatism of identical age to the and Recycled, but a plume signature is not recognized. Mashak Igneous event, especially to the most precise The second event, the Mashak igneous event consists age estimate of 1385 Ma for the Bakal dyke (Fig. 19) age of Middle Riphean volcanic suites and geochemically obtained in this study has also been found in western correlated dykes. A precise U–Pb baddeleyite age of Laurentia (Hart River sills and Salmon River Arch sills), 1385.3±1.4 Ma is obtained herein for the Bakal quarry in northern Greenland (Midsommerso sills and ZigHZag dyke. This age provides a precise link to Mashak magmatism Dal volcanics), in the Anabar Shield of Siberia (Chieress and represents the most precise estimate available for dykes), in Antarctica (Vestfold Hills dykes), and in the the Mashak igneous event. Despite geological and Congo craton (see reviews in Ernst and Buchan, 2001; structural evidence for an extensional setting, some Ernst et al., 2006). In the Rodinia reconstruction, Baltica geochemistry indicates a subduction component was adjacent to northeastern Laurentia at 1265 Ma based (moderate La–Nb anomaly), although this could also

157 Институт геологии Уфимского научного центра РАН be obtained from involvement of lithospheric mantle. out a link with event 3 and reaffirms that similar Other diagrams indicate a MORB or BackHArc setting. geochemical fingerprints can sometimes be generated This may further point to involvement of lithospheric by more than one event. The mantle source is dominated mantle. The setting is within plate (but could also be by a Recycled component, and a plume is implied. back arc). There is no subduction signature. Mantle The fifth event comprises felsic volcanics of the sources are Depleted with substantial contributions of Arsha formation of Vendian age. Geochemistry suggests Enriched and Recycled sources. Plume source is likely. a within plate setting (no subduction signature). The mantle The third magmatic event of unknown age comprises source is Depleted with minor Recycled and Enriched dykes and sills cutting preHRiphean basement and a high components, and plume involvement is equivocal. Mg sill intruding the lower Riphean Bakal formation. To summarize, the first two events represent early An arc setting is strongly suggested by a consistent calcH Riphean and middle Riphean rifting events. Event 3 alkali signature on a variety of diagrams, and large negative could suggest a calcHalkaline arc. The renewed rifting of Ta–Nb anomaly. This event shows strong arc signature events 4 and 5 took place possibly in the Upper Riphean which matches the strong subduction signature. SubH and certainly in the Vendian. equal Depleted and Enriched components were involved It is possible that the subduction signature (high in the source, and do not match a plume source. La/Nb) of the observed events reflects lithospheric or The fourth event comprises mafic dykes which cut crustal contamination, which was strongest in the Lower upper Riphean formations and which define a compositional Riphean (event 1, and event 3?), weaker in the middle grouping distinct from Arsha volcanics (below), with Riphean (event 2) and absent in the younger upper Riphean withinHplate character. Geochemically, this event is similar and Vendian units (events 4 and 5). to the much older ca. 1380 Ma Mashak (event 3), although Most magmatism is mafic, but felsic rocks are also it lacks a subduction signature. The age difference rules present. In the Ai formation mafic and felsic suites are

Figure 19. Geochronology of Bakal dyke (see the explanations in the text)

158 Геологический сборник № 5. Информационные материалы cogenetic, while in the Mashak formation, the mafic control is very poor, and they could be coeval. It is worth and felsic rocks have distinct geochemical patterns. mentioning that the stage of rift volcanism and intrusive The felsic rocks may represent melting of continental activity in the Kvarkush anticlinorium of the Urals, crust by the basaltic component of the event. The Vendian situated to the north of the Bashkirian meganticlinorium, (felsic) volcanics have distinct chemistry from upper encompasses both the Upper Riphean and Vendian Riphean – lower Vendian (or younger) dykes, but age (Petrov et al., 2005).

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Figure 20. Reconstruction of Baltica and Laurentia at 1267 Ma (after Buchan et al., 2000). Location of pre1Timanian margin inferred from distribution of “Timanian age complexes” from Fig. 1 in Pease et al. (2006)

Cabanis, B., Lecolle, M. 1989. Le diagramme La/10– Y/15–Nb/8: un outil pour la discrimination des series volcaniques et la mise en evidence des processus de me′ lange et/ou de contamination crustale. Comptes Rendus de l’Academie des Sciences, Series II, v. 309: 2023–2029. Condie, K.C. 1997. Sources of Proterozoic mafic dyke swarms: constraints from Th/Ta and La/Yb ratios. Precambrian Research, v. 81: 3–14. Condie K.C. 2001. Mantle Plumes and Their Record in Earth History. Cambridge Univ. Press, Oxford, U.K., 306 p. Condie, K.C. 2003. Incompatible element ratios in oceanic It is proposed that the Mashak event and the basalts and komatiites: Tracking deep mantle sources and Midsommerso — ZigHZag Dal event of Greenland continental growth rates with time. Geochemistry, Geophysics, are part of the same event which caused rifting along Geosystems. v. 4, no. 1, 1005, doi: 10.1029/2002GC000333. the north eastern margin of Baltica at 1380 Ma. Ellmies R., Glodny J., Krupenin M. 2000. A metallogenic The conjugate margin could be any of the other blocks model for the sedimentHhosted deposits of the Proterozoic with 1380 Ma events such as: Siberia, or Antarctica Bashkir basin // Geological Survey and mineral deposits of (Ernst and Buchan, 2001). Russia at the verge of the XX century. V. 2. Mineral reserves of Russia. Mat. of the AllHRussian Meeting of geologists StH Petersburg, VSEGEI: 6–7(in Russian). Ernst, R.E., Wingate, M.T.D., Buchan, K.L., Li, Z.X. ACKNOWLEDGEMENTS (in prep). Global Record of Large Igneous Provinces (LIPs) during the life cycle (1600–700 Ma) of the proposed supercontinent The material was collected during the field excursion Rodinia Precambrian Research (Special issue on Rodinia). in 2003, organized and sponsored by IGSP 440 «Rodinia». Fitton, J.G., Saunders, A.D., Norry, M.J., Hardarson, B.S., The authors also gratefully acknowledge financial Taylor, R.N. 1997. Thermal and chemical structure of the Iceland support from BHP–Billiton company for the U–Pb plume. Earth Planet. Sci. Lett. 153: 197–208. geochronology done by MH at the University of Toronto Garan M.I. 1969. The Lower and Middle Precambrian. laboratory, the Swedish National Science Research Geology of USSR. Permian, Sverdlovsk, and Kurgan Council, the Swedish Royal Academy of Sciences, the oblast. V. 12, part 1 (1). W, Nedra: 64–149 (in Russian). Earth Sciences Department of Russian Academy of Glasmacher, U.A., Bauer, W., Giese, U., Reynolds, P., Sciences, Program 10 “Central Asian Mobile Belt: Kober, B., Puchkov, V., Stroink, L., Alekseyev, A., Willner, A.P. 2001. The metamorphic complex of Beloretzk, SW Urals, Russia — geodynamics and stages of the Earth’s crust formation”. a terrane with a polyphase MesoH to Neoproterozoic thermoH IGPET 2000 was used for plotting geochemical data. dynamic evolution. Precambrian Research, 110: 185–213. Gorozhanin V.M. 1995. The Rb–Sr isotopic method in REFERENCES: solving problems of geology of the Southern Urals. Autoreferate of thesis of candidate of geol.Hmin. sci. Ekaterinburg, 23 p. Alekseyev, A.A. 1984. RipheanHVendian magmatism of (in Russian). the western slopes of the Southern Urals. M. Nauka, 136 p (in Gradstein F.M., Ogg J.G., Smith A., Bleeker W, Lourens L.J. Russian). 2004. A new geologic Time Scale, with special reference to Becker, H., Jochum, K., Carlson, R. 2000. Trace element Precambrian and Neogene/ Episodes, V. 27, No 2: 83–100. fractionation during dehydration of eclogites from highH Grachev A.F. 1977. Rift zones of the Earth. Leningrad, pressure terranes and the implications for element fluxes in Nedra. 247 p. (in Russian). subduction zones. Chemical Geology, 163: 65–99. Buchan, K.L., Mertanen, S., Park, R.G., Pesonen, L.J., Hart, S.R. 1988. Heterogeneous mantle domains: Elming, S.TA°., Abrahamsen, N., Bylund, G. 2000. Comparing signatures, genesis and mixing chronologies. Earth. Planet. the drift of Laurentia and Baltica in the Proterozoic: the Sci. Lett. 90: 273–296. importance of key paleomagnetic poles. Tectonophysics, 319: Hart, S.R., Hauri, E.H., Oschmann, L.A., Whitehead, J.A. 167–198. 1992. Mantle plumes and entrainment: isotopic evidence. Bryan, S.E., Riley, T.R., Jerram, D.A., Leat, P.T., Science 256: 517–520. Stephens, C.J. 2002. Silicic volcanism: an underHvalued Herzberg, C. 1995. Generation of plume magmas through component of large igneous provinces and volcanic rifted time: An experimental approach. Chemical Geology 126: 1–16. margins In: Menzies, M.A., Klemperer, S.L., Ebinger, C.J., Irvine, T.N., Baragar, W.R.A. 1971. A guide to the chemical Baker J. (eds.) Magmatic Rifted Margins. Geological Society classification of the common volcanic rocks. Can. J. Earth Sci. 8: of America Special Paper 362: 99–120. 523–548. 160 Геологический сборник № 5. Информационные материалы

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